Well, first off, it wouldn't be the first inter-religious couple ever. It happens.
What if? I personally don't think much on it. They're young and obviously not basing their relationship on religion which means neither are as deeply rooted in their religion as the parents want them to be if they are against it. Should the parents interfere too heavily, they will likely end up the bad guys whereas if they let it take it's course, it will likely end on its own. If not, then perhaps the parents need to reconsider their stance on what love is made from.
With my children, I'm much more concerned with their happiness, treatment and wisdom they use while in "young love".